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From: carl@CITHEX.CALTECH.EDU.UUCP
Newsgroups: comp.os.vms
Subject: Re: VMS CHECKSUM algorithm
Message-ID: <870628092732.00f@CitHex.Caltech.Edu>
Date: Sun, 28-Jun-87 12:43:00 EDT
Article-I.D.: CitHex.870628092732.00f
Posted: Sun Jun 28 12:43:00 1987
Date-Received: Tue, 30-Jun-87 01:11:08 EDT
References: <3135@eagle.ukc.ac.uk>
Sender: daemon@ucbvax.BERKELEY.EDU
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Lines: 29


 > Anyone know the  algoritm  used  by  the  undocumented  CHECKSUM  command  on
 > VAX/VMS?  I'd like to be able to generate/check these on non VAX/VMS systems,
 > to allow some measure of checking on transferred files.

I assume that you only need the algorithm for CHECKSUM/FILE, and not  the  stuff
for  CHECKSUM/IMAGE.   If this is the case, then what you need to know (assuming
that you've opened the file with RMS, and that your RAB is called "myrab") is:

#include rab
#include rms
unsigned long *longbuffer;
unsigned char *charbuffer;
long n_longwords, n_bytes, i;
checksum = 0;
while ((status = SYS$GET(myrab)) != rms$_eof)
{	if ((status & 7) != 1)
	{	puts("Error during read:");
		exit(status);
	}
	n_longwords = myrab.rab$w_rsz/4;
	n_bytes = myrab.rab$w_rsz & 3;
	longbuffer = (unsigned long *) &(myrab.rab$l_rbf);
	bytebuffer = (unsigned char *) &(longbuffer[n_longwords]);
	for (i = 0; i < n_longwords; ++i)
		checksum = checksum XOR longbuffer[i];
	for (i = 0; i < n_bytes; ++i)
		checksum = checksum + bytebuffer[i];
}